Fish
Aquarium calculator: gallons, litres, weight and heater
Multiply length × width × height in inches and divide by 231 for US gallons (or cm³ ÷ 1,000 for litres): a 48 × 13 × 21 in tank holds about 57 gallons, around 49 with a gap at the top and gravel.
Aquarium size chart: standard tanks
Common US glass tank sizes. “Water” assumes a 1-inch gap at the top and 2 inches of gravel; weight is water only (filled to 1 inch below the rim).
| Tank | L × W × H (in) | Gallons | Litres | Water (gal) | Water weight (lb) | Heater |
|---|---|---|---|---|---|---|
| 5.5 gallon | 16 × 8 × 10 | 5.5 | 21 | 3.9 | 42 | 12–19 W |
| 10 gallon | 20 × 10 × 12 | 10.4 | 39 | 7.8 | 79 | 23–39 W |
| 15 gallon | 24 × 12 × 12 | 15 | 57 | 11.2 | 114 | 34–56 W |
| 20 high | 24 × 12 × 16 | 19.9 | 76 | 16.2 | 156 | 49–81 W |
| 20 long | 30 × 12 × 12 | 18.7 | 71 | 14 | 143 | 42–70 W |
| 29 gallon | 30 × 12 × 18 | 28.1 | 106 | 23.4 | 221 | 70–117 W |
| 40 breeder | 36 × 18 × 16 | 44.9 | 170 | 36.5 | 351 | 109–182 W |
| 55 gallon | 48 × 13 × 21 | 56.7 | 215 | 48.6 | 451 | 146–243 W |
| 75 gallon | 48 × 18 × 21 | 78.5 | 297 | 67.3 | 624 | 202–337 W |
| 90 gallon | 48 × 18 × 24 | 89.8 | 340 | 78.5 | 717 | 236–393 W |
| 125 gallon | 72 × 18 × 21 | 117.8 | 446 | 101 | 936 | 303–505 W |
Outside dimensions vary by maker by up to an inch; glass thickness takes a little off the inside.
Citing this? Link to https://tools.thesturdypaw.com/aquarium/. Volumes are geometry: in³ ÷ 231 = US gallons. Free to reuse with a link (CC BY 4.0).
The formulas
- Rectangle: L × W × H in inches ÷ 231 = US gallons; in cm, L × W × H ÷ 1,000 = litres.
- Cylinder: π × (diameter ÷ 2)² × height.
- Hexagon: 2.598 × side² × height.
- Bow front: the rectangle plus the curved segment at the front.
- Conversions: 1 US gallon = 3.785 litres = 0.833 UK gallons; fresh water weighs about 8.34 lb per US gallon.
Questions
How do I calculate aquarium gallons?
Multiply length × width × height in inches and divide by 231: a 48 × 13 × 21 in tank is 56.7 gallons, and about 48.6 gallons of actual water with a 1-inch gap and 2 inches of gravel.
How much does a 55-gallon fish tank weigh?
The water alone weighs about 451 lb when filled to an inch below the rim; add the glass, stand, gravel and rock, so plan for well over 500 lb on the floor.
What size heater does my aquarium need?
The common rule is 3 to 5 watts per gallon for a tropical tank, using the higher end in a cold room or a large temperature gap.
How many fish can I put in my tank?
The old “one inch of fish per gallon” rule only suits small, slim fish; filtration, adult size and species behaviour matter more. Use it as a ceiling for tiny fish, not a target.
How powerful a filter do I need?
Look for a filter rated at about 4 to 6 times the tank’s volume per hour: 200 to 300 gallons per hour for a 50-gallon tank.
How many litres is a 20-gallon tank?
About 76 litres (US gallons). A UK gallon is bigger: 20 UK gallons is about 91 litres.
How many gallons is a 48 × 18 × 21 inch tank?
About 78.5 gallons: 48 × 18 × 21 = 18,144 cubic inches, divided by 231.
How do I convert litres to gallons?
Divide litres by 3.785 for US gallons (or by 4.546 for UK gallons): 100 litres is about 26.4 US gallons.
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Sources
- Volume and weight are geometry and unit conversion: 231 in³ per US gallon, 3.785 L per US gallon, 8.34 lb per gallon of fresh water.